This invention relates to electronic circuits, and in particular to opto-electronic circuits forming part of the sensor component in linear or rotary encoders for the measurement of linear or angular displacement respectively.
Typically in such encoders, one or more fixed sources of electromagnetic radiation (EMR) are arranged to illuminate a graduated planar or cylindrical surface. The markings on the graduated surface comprise regions of high and low reflectivity (or, alternatively, high and low transmissibility) to the EMR and the reflected (or transmitted) component of the EMR is arranged to impinge on a fixed opto-electronic sensor which detects the time-dependent or spatially distributed intensity pattern of the incident EMR, and hence provide measurement of the relative position of the graduated surface. Both such xe2x80x9creflectivexe2x80x9d and xe2x80x9ctransmissivexe2x80x9d encoder versions are used commonly in industry and consumer products today for measurement of linear or angular displacement, although the latter arrangement is more common.
The opto-electronic sensor incorporated in such encoders typically employ four photodiodes and a xe2x80x9cquadrature interpolationxe2x80x9d method, well known in the art, is used to increase the positional measurement resolution well above the pitch of markings on the respective graduated planar or cylindrical surface of the encoder. In U.S. Pat. No. 4,410,798 (Breslow) and U.S. Pat. No. 5,235,181 (Durana et al.) various implementations are described where the measurement resolution is increased many times higher than the pitch of the markings, being limited by the accuracy of the marking pitch, width and edge quality, the positional accuracy of photodiodes providing the quadrature signals and the signal to noise ratio of these photodiodes. The disadvantage of these systems are, that in order to provide measurement accuracy in the order of microns, very high quality components, xe2x80x9cmicron accuracyxe2x80x9d mechanical and assembly tolerances, and high quality markings are needed. As an example, in both the above prior art patents, the phase error of the quadrature signals is equal to the positional error of the discrete photodiode detectors, plus the positional error of the markings, plus the errors due to the electrical and optical noise, mismatch of the detectors, and other components in the signal processing system.
The electronic circuit according to the present invention seeks to overcome some of these disadvantages by extensive over-sampling of the incident EMR via an opto-electronic sensor which has one or more arrays of multiple photodiode detectors, each array of photo-diode detectors simultaneously spanning many pitches of the pattern, of incident EMR impinging on the array. In this specification xe2x80x9cpitchxe2x80x9d of the pattern is defined as the distance between adjacent regions of maximum EMR intensity of the pattern of incident EMR impinging on the array of detectors, and directly relates to the pitch of the markings on the graduated surface or surfaces from which the EMR was reflected (for a reflective encoder) or through which the EMR was transmitted (for a transmissive encoder). As a result the measurement accuracy is higher than the positional accuracy of any single detector in the array, and indeed the positional accuracy of a single pattern pitch. The positional accuracy of the resulting quadrature pair signals is not determined by mechanical and assembly tolerances, but by the positional accuracy of the xe2x80x9cvery large scale integrationxe2x80x9d (VLSI) process used for the manufacture of the photodiode arrays and can be as low as 0.1 um economically with today""s silicon fabrication processes. According to the present invention, the opto-electronic circuit for the encoder sensor component does not rely on the use of expensive and highly accurate marking processes for the manufacture of the encoder, and is able to tolerate local imperfections in the graduated surface(s) of the encoder, even damaged or entirely missing areas of markings.
It is an aim of this invention to provide a very accurate opto-electronic relative position measurement circuit comprising less accurate (and hence lower cost) elements. A sensor component of an encoder, employing an opto-electronic circuit according to the present invention, will typically have  greater than 100 times higher resolution than the pitch of the markings on the graduated surface whose relative position is to be measured,  greater than 10 times higher resolution than the pitch of the array of photodiode detectors, and  greater than 10 times higher measurement accuracy than the positional accuracy of any of the individual detectors in the array.
Extensive over-sampling of the incident EMR pattern impinging on the array, and massively-parallel collective computation within the opto-electronic circuit, results in the relative position measurement resolution being determined by the pitching accuracy of the detector array, rather than being limited by the pitching accuracy of the encoder marking graduations. In addition, by measuring many pitches of the incident EMR pattern, the accuracy of the relative position measurement is theoretically np times higher than the positional accuracy of any individual pitch of the incident EMR pattern, where np is the number of pattern pitches being sampled by the detector array. The advantage of this approach is that it does not require an expensive graduation marking process to achieve xe2x80x9csubmicronxe2x80x9d resolution in relative position measurement of the sensor component. Furthermore, assuming analog (for example photodiode) detectors are used in the array, the measurement technique provides subpixel resolution, and can economically supply 10-100 times higher resolution than that of the detector array pitch. The signal to noise ratio of the relative position measurement is typically more than nd times larger than the signal to noise ratio of the individual detectors, where nd is the number of detectors in the array. This is a clear advantage because, using today""s VLSI silicon fabrication processes, one or more arrays consisting of thousands of detectors can be implemented economically on a single chip.
The computation that is needed for the relative position measurement is mainly an inner-product operation, which is executed very efficiently by a capacitive circuit. The capacitive correlator circuits, according to the present invention, carry out the necessary computation in a massively-parallel single operation, achieving the highest possible processing speed and hence minimum processing time. The computing accuracy is related to the accuracy of the capacitors within the capacitive correlator circuits which is the best controlled parameter of VLSI circuits, providing the most area-efficient solution for the type of circuit architecture. Also the massively-parallel, analog inner-product computation block minimizes the energy needed to perform the computation, requiring as little as 0.1% of the power dissipation of corresponding digital microprocessor solutions.
The electronic circuit, according to the present invention, is suitable for very high resolution relative position measurement over a range corresponding to one pitch of the incident EMR pattern. To obtain high resolution absolute position measurement extending beyond this one-pitch range, the circuit can be used in combination with simple, lower resolution absolute bar code measurement techniques. For example, the graduation markings may be encrypted using a variable width marking (eg. a binary thin/thick marking) to the graduations or, alternatively, a separate bar-code marking graduation on the planar or cylindrical surface in the encoder can be employed. Many different bar-code marking encryption techniques are commonly used in industry and consumer products.
The present invention consists in an electronic circuit comprising a longitudinally disposed array of electromagnetic radiation (EMR) detectors, two or more correlator units, and a phase angle computing unit, the circuit enabling measurement of the relative position of a spatially periodic intensity pattern of incident EMR impinging on the array of detectors, characterised in that the pitch of the array of EMR detectors is arranged to be smaller than the pitch of the spatially periodic intensity pattern of incident EMR, each correlator unit comprises an array of capacitors connected to a buffer, each detector has an output dependent on the incident EMR impinging on that detector, the output communicated to respective one or more capacitors in each of the two or more correlator units, the capacitances of the one or more capacitors determine a correlator coefficient for that detector in relation to the respective correlator unit, the magnitude of the correlator coefficients arranged to vary periodically in the longitudinal direction along the length of the array according to a predetermined periodic weighting function, each correlator unit analog-computing the weighted sum of the respective detector outputs according to the respective predetermined periodic weighting function for that correlator unit and outputting an analog representation of this weighted sum at its respective buffer, the weighting functions of the two or more correlator units mutually offset by a predetermined phase angle, the phase angle computing unit connected to the buffers of each correlator unit, and enabling computation of the relative phase angle of the spatially periodic intensity pattern of incident EMR, and hence its relative position expressed as the relative phase angle of the pattern.
It is preferred that each array of capacitors of each correlator unit comprises a first and second capacitor sub-array, each of the capacitor sub-arrays comprise a common plate and a plurality of top plates, the common plate of the first capacitor sub-array is connected to the positive input of the buffer of the correlator unit, the common plate of the second capacitor sub-array is connected to the negative input of the buffer of the correlator unit, and each detector is connected to the top plate of the respective capacitor of the first capacitor sub-array of the correlator unit and to the top plate of the respective capacitor of the second capacitor sub-array of the correlator unit.
It is preferred that the output of each detector is a voltage, the detector voltage output is applied as an input to the respective capacitor of each of the first and second capacitor sub-arrays of each of the two or more correlator units via an array of switches, each switch having at least two states comprising a calibration state wherein the respective capacitor top plates are connected to a reference voltage and the buffer output is set to zero, and a functional state wherein the respective capacitor top plates are connected to the detector output voltage.
It is preferred that the input applied to the capacitor of the first capacitor sub-array is equal to the input applied to the capacitor of the second capacitor array for each correlator unit, the input comprises a voltage transition from a predefined reference voltage to the respective detector output voltage, and the correlator coefficients for each detector in relation to the respective correlator unit is therefore the difference of the capacitances of the respective first and second capacitor sub-array capacitors.
Alternatively, it is preferred that, for each correlator unit, a positive correlator coefficient is generated if the input to a given capacitor of the first capacitor sub-array is a voltage transition from a predefined reference voltage to the respective detector output voltage and the input to the respective capacitor of the second capacitor sub-array is a voltage transition from the respective detector output voltage to the predefined reference voltage, a negative correlator coefficient is generated if the input to a given capacitor of the first capacitor sub-array is a voltage transition from the respective detector output voltage to the predefined reference voltage and the input to the respective capacitor of the second capacitor sub-array is a voltage transition from the predefined reference voltage to the respective detector output voltage, and the absolute value of the correlator coefficient is therefore the sum of the capacitances of the respective first and second capacitor sub-array capacitors.
It is preferred that the electronic circuit comprises two correlator units.
It is preferred that the correlator coefficients vary sinusoidally along the length of the array, having a pitch equal to the pitch of the spatially periodic intensity pattern of the incident EMR, the first correlator unit having a phase angle of zero degrees and calculating the sine weighted sum of the detector outputs, and the second correlator unit having a phase angle of 90 degrees and calculating the cosine weighted sum of the detector outputs.
It is preferred that the capacitor top plates have equal width measured in the longitudinal direction of the array and a varying length measured perpendicular to this direction.
Alternatively, it is preferred that the capacitor top plates have equal length and varying width.
It is preferred that the pitch of the array of detectors and the weighting functions of each of the correlator units are arranged such that each of the weighting functions are zero halfway between different adjacent pairs of detectors of the array.
It is preferred that the pitch of the weighting functions of each of the correlator units are arranged to be equal to the pitch of the spatially periodic intensity pattern of the incident EMR.
It is preferred that the electronic circuit is an integrated circuit.